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Message-ID: <20141002102723.GB23661@linutronix.de>
Date: Thu, 2 Oct 2014 12:27:23 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Frans Klaver <frans.klaver@...ns.com>
Cc: Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
Peter Hurley <peter@...leysoftware.com>, tony@...mide.com,
gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
balbi@...com, linux-serial@...r.kernel.org,
linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround
for AM33xx
* Frans Klaver | 2014-09-30 10:44:16 [+0200]:
>On Mon, Sep 29, 2014 at 12:30:43PM +0200, Frans Klaver wrote:
>> On Mon, Sep 29, 2014 at 11:54:40AM +0200, Sebastian Andrzej Siewior wrote:
>> > For your "too much work for irq" problem: Could you add trace_printk()
>> > in tx/rx dma start/complete, and irq routine? The interresting part is
>> > what is the irq routine doing once entered. It might be a condition that
>> > is ignored at first and "acked" later while serving another event. Or it
>> > is really doing something and this is more or less "legal".
>>
>
>Here's some trace output I get. I hope branches become clear by the
>calls they do.
>
> uart_test-482 [000] .ns. 17.860139: __dma_rx_do_complete: error: 0, uart_bug_dma: 32
> uart_test-482 [000] .ns. 17.860141: __dma_rx_do_complete: rx_dma(p, 0)
these two happen outside the IRQ routine for every 48 bytes.
…
> uart_test-483 [000] dnh. 17.861018: serial8250_interrupt: irq 89
> uart_test-483 [000] dnh. 17.861026: serial8250_interrupt: 89 e
e? Did was the routine invoked 0xe times or this a register?
> uart_test-483 [000] .ns. 17.861045: __dma_rx_do_complete: error: 0, uart_bug_dma: 32
> uart_test-483 [000] .ns. 17.861047: __dma_rx_do_complete: rx_dma(p, 0)
another 48bytes
> uart_test-479 [000] d.H. 17.861124: serial8250_interrupt: irq 89
> uart_test-479 [000] d.H. 17.861133: serial8250_handle_irq: l1 IIR cc LSR 61
> uart_test-479 [000] d.H. 17.861135: serial8250_handle_irq: rx_dma(up, iir)
> uart_test-479 [000] d.H. 17.861139: serial8250_rx_dma: l1, UART_IIR_RX_TIMEOUT
> uart_test-479 [000] d.H. 17.861147: __dma_rx_do_complete: error: 1, uart_bug_dma: 32
> uart_test-479 [000] d.H. 17.861150: serial8250_handle_irq: rx_chars(up, status)
> uart_test-479 [000] d.H. 17.861190: serial8250_handle_irq: rx_dma(up, 0)
timeout, manual purge. Do you have an idea how many bytes were manually
received?
> uart_test-479 [000] d.H. 17.861205: serial8250_interrupt: 89 e
> kworker/0:1-10 [000] dnH. 17.864949: serial8250_interrupt: irq 89
>
>So far so good. We're just entered interrupt where stuff goes awry. The
>following pattern repeats over 600 times:
>
> kworker/0:1-10 [000] dnH. 17.865198: serial8250_handle_irq: l1 IIR cc LSR 61
> kworker/0:1-10 [000] dnH. 17.865254: serial8250_handle_irq: rx_dma(up, iir)
> kworker/0:1-10 [000] dnH. 17.865333: serial8250_rx_dma: l1, UART_IIR_RX_TIMEOUT
> kworker/0:1-10 [000] dnH. 17.865626: __dma_rx_do_complete: error: 1, uart_bug_dma: 32
> kworker/0:1-10 [000] dnH. 17.865747: serial8250_handle_irq: rx_chars(up, status)
> kworker/0:1-10 [000] dnH. 17.868797: serial8250_handle_irq: rx_dma(up, 0)
>
>ending with:
>
> kworker/0:1-10 [000] dnH. 20.027093: serial8250_interrupt: serial8250: too much work for irq89
> kworker/0:1-10 [000] dnH. 20.027181: serial8250_interrupt: 89 e
>
>This clogs the entire system until I disconnect the communication lines.
>
>So we get an RX timeout. The receiver fifo trigger level was not reached
>and 8250_core is left to copy the remaining data. I would expect that if
>the trigger level wasn't reached, we wouldn't need to be doing this that
>often. On the other hand, could we be trapped reading data from rx
>without dma helping us? And how could we resolve this?
So if I understand you correct, then you enter serial8250_interrupt()
and then you enter multiple times omap_8250_dma_handle_irq() and
you always get a TIMEOUT event and fetch byte(s) manualy via
serial8250_rx_chars(). And after one iteration UART_IIR_NO_INT is not
set and you do it again, right?
I have no idea when exactly the timeout-interrupt fires. The manual
says: "… the count is reset when there is activity on uarti_rx …" but it
does not say how often it increments before the level is reached.
It also might be that you have a small gap between two bytes and this
high baud rate the gap is considered as a timeout event.
Another thing could be that if the we enqueue a RX transfer from the
dma_completion callback then we have too many bytes in the FIFO already
becahse the callback is invoked from softirq. What happens if we cut the
middle man via
diff --git a/drivers/dma/virt-dma.c b/drivers/dma/virt-dma.c
index 6f80432..21b04bd 100644
--- a/drivers/dma/virt-dma.c
+++ b/drivers/dma/virt-dma.c
@@ -63,8 +63,9 @@ static void vchan_complete(unsigned long arg)
dma_async_tx_callback cb = NULL;
void *cb_data = NULL;
LIST_HEAD(head);
+ unsigned long flags;
- spin_lock_irq(&vc->lock);
+ spin_lock_irqsave(&vc->lock, flags);
list_splice_tail_init(&vc->desc_completed, &head);
vd = vc->cyclic;
if (vd) {
@@ -72,7 +73,7 @@ static void vchan_complete(unsigned long arg)
cb = vd->tx.callback;
cb_data = vd->tx.callback_param;
}
- spin_unlock_irq(&vc->lock);
+ spin_unlock_irqrestore(&vc->lock, flags);
if (cb)
cb(cb_data);
diff --git a/drivers/dma/virt-dma.h b/drivers/dma/virt-dma.h
index 181b952..7632338 100644
--- a/drivers/dma/virt-dma.h
+++ b/drivers/dma/virt-dma.h
@@ -92,7 +92,10 @@ static inline void vchan_cookie_complete(struct virt_dma_desc *vd)
vd, cookie);
list_add_tail(&vd->node, &vc->desc_completed);
- tasklet_schedule(&vc->task);
+ if (vd->tx.my_uart)
+ vc->task.func(vc);
+ else
+ tasklet_schedule(&vc->task);
}
/**
diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 57a8b12..5d7ee92 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -728,6 +728,7 @@ static int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
dma->rx_running = 1;
desc->callback = __dma_rx_complete;
desc->callback_param = p;
+ desc->my_uart = 1;
dma->rx_cookie = dmaengine_submit(desc);
diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 1f9e642..0f5fbe1 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -459,6 +459,7 @@ struct dmaengine_unmap_data {
struct dma_async_tx_descriptor {
dma_cookie_t cookie;
enum dma_ctrl_flags flags; /* not a 'long' to pack with cookie */
+ u32 my_uart;
dma_addr_t phys;
struct dma_chan *chan;
dma_cookie_t (*tx_submit)(struct dma_async_tx_descriptor *tx);
>
>Frans
Sebastian
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